1. Field of the invention
The invention concerns the temporary locking onto a stator of a rotor that normally rotates on the stator on magnetic bearings. It is particularly, but not exclusively, concerned with locking an inertia wheel of a space satellite in position.
2. Description of the prior art
When a satellite is launched and injected into its orbit, it is subjected to varied and high mechanical loads. These subject any inertia wheel on board the satellite to sinusiodal and/or random vibration on each axis (rotation axis and two transverse axes completing a reference trihedron).
Because of their magnitude these loads cannot be satisfactorily absorbed by the magnetic (and therefore mechanical contact-free) bearings. It is therefore known to temporarily lock the inertia wheel to the remainder of the satellite structure forming a stator, by mechanical means, during the launching of the satellite. Locking systems designed to this end must prevent any problems of binding due to rubbing and/or jamming which may occur at the time of release in the space type but nevertheless severe environment.
A first locking system designed for this purpose described by ESTEC in "Experimental Caging Mechanism" (cf. Gen. Arrgt. Drg No. 1 300, Oct. 1982) resembles a drum brake with two jaws which are spread apart radially and pressed against an internal cylindrical portion at the periphery of the rotor, due to the mechanical action of a circumferential cable the tension in which is transferred to the jaws by two sets of links. In the locked configuration the rotor is immobilized by the tangential friction exerted on it by the jaws; to release the rotor the locking cable is cut by a pyrotechnic cable cutter and return springs pull the jaws to a radially retracted configuration.
For such locking by tangential friction to be effective, the locking cable must be at a high tension and remain at this high tension in spite of relaxation phenomena which occur in it. Also, the locking forces are very unevenly distributed since they are located at the diametrally opposed jaws. Furthermore, the path of the cable is relatively complex, departing from a simple path near the jaws and having very small radii of curvature; although these small radius of curvature areas can be sheated, they are inevitably subjected to friction which is prejudicial both to correct tensioning of the cables at the jaws when locked and to clean releasing of the jaws when the cable is cut; finally, no redundancy is provided in the cable release means.
In a second known locking system (cf. French Pat. No. 2.549.598) developed by AEROSPATIALE, the rotor is bell-shaped at its periphery and annular chamfers are formed on the stator and on the rotor, near the periphery of the latter, adapted to come into axial bearing engagement due to the action of a central cable disposed along the rotation axis of the rotor and tensioned to effect the locking. Release is similarly effected by cutting the cable using a pyrotechnic device.
A system of this kind exerts the locking forces at the center, that is to say at a location where the moments of inertia are the lowest, which can result in the rotor lifting off the stator at the periphery. Good locking requires a high tension in the cable, as previously. Finally, in this system there is no mechanical redundancy in the cable release means.
In a third type of locking device, also developed by AEROSPATIALE, a clamp fastens together two axially opposed flanges attached to the rotor and the stator, respectively. This clamp is formed by two jaws each articulated at one end about an axis parallel to the rotation axis of the rotor and subtending an angle of slightly less than 180.degree. around the flanges; their other ends are joined by a tensioned cable which tends to pull them towards each other against a separator spring around the cable. Two grooves are formed on the radially inside wall of these jaws to accommodate the aforementioned flanges. Cable release is effected by a pyrotechnic cable cutter and the jaws are forced apart by the compressed spring.
A solution of this kind is very costly because of the need to shape the grooves to the exact complementary profile of the flanges. Also, locking requires a very high tension in the cable which generally results in deformation of the jaws. Once again their is no mechanical redundancy.
In a fourth type of locking device, also developed by AEROSPATIALE, disclosed in French Pat. No. 2.452.693 and U.S. Pat. No. 4.345.485, locking is effected along only three axes, meaning that the rotor is not immobilized relative to the stator in rotation. Locking is effected by temporarily cancelling the axial clearance which normally exists at the ends of a central shaft of the rotor between axial bearing surfaces provided on the rotor and on the stator. The axial clearance is eliminated by rendering an axial bearing surface on the stator axially mobile towards the corresponding bearing surface on the rotor, by virtue of radial movement of a shim forming a ramp due to the action of a cable disposed radially and against the action of a return spring. A cable cutter is provided to perform the release function.
This solution is somewhat complex as it presupposes the provision on the rotor or on the stator near axial bearing surfaces of bearings adapted to withstand high dynamic axial loads to enable rotation of the rotor in the locked configuration. Also, the various sliding movements that occur produce friction leading to the risk of binding. Finally, there is no mechanical redundancy in the release means.
These known solutions give satisfaction but an object of the invention is to remedy their disadvantages. It is particularly directed to a cable-type locking device adapted to operate efficiently and homogeneously at the periphery of the rotor without requiring the tension in the cable to be either very high or perfectly controlled, and which involves only minimum friction on releasing, so that release is clean.